While I haven't dealt with them directly on Vista or 7, with XP, if there was "a significant" hardware change, you would be prompted to call. Case in point, through several years of being a local computer shop tech, several customer brought in Compaqs, Dells, etc. that had board problems or that wanted a general upgrade and transfer of components from old to new. We would load their OS with the Dell provided key, which is OEM of course, and be prompted to call for activation. Call in, and suggest to the MS rep that the system board died and this computer is replacing the previous system and continuing the use of the OS, and they'll promptly activate your software and you'll be on your way. At least, that's been my experience. As far as activations go, I can't see it being too ethical to refuse to activate someone's product because of a simple hardware change. I can see them refusing to activate a product that has had 10 or more activations in less than a month, as that would lead to the idea of piracy.